Walkthrough

Urag in the College of Winterhold starts this quest. Read the book, then speak to him to be sent on a quest to locate Septimus in an isolated area north of the College. There is a path leading down and across various icebergs. Once you've spoken to the loopy old man, you will need to start heading to some Dwemer ruins.

The inside of the ruins is rife with traps and mechanical enemies: Dwarven Spiders and Dwarven Spheres, to be exact. If you ever get lost navigating this huge area, just remember that the idea is to head deeper into the subterranean level, where you eventually come upon Falmer enemies that can poison you.

When you reach a gated area with seemingly no lever in immediate sight, you need to head behind and up the stairs to the altar overlooking the gate to unlock it. There, interact with the Dwarven mechanism to open another way through to Blackreach. Meander through Blackreach to fight off some more Falmer and a Dwarven Centurion, going deeper yet to a lift that takes you to the Tower of Mzark.

It is there that you finally place the Lexicon into the slot next to the beacon for the huge mechanism in the room. Press the left button of the two available until the a third button lights up on the left. Press that until the last button lights up, then take the Runed Lexicon from the slot.

Head back to Septimus Sigmus now, using an elevator short-cut in Blackreach. The quest continues if you are level 15 or above; if not, grind your way up and wait for a courier to deliver a message, prompting you to return to Septimus.